While a number of efforts are currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology.
One technology that has attracted a good deal of attention recently utilizes as the deep UV light source a high-intensity KrF excimer laser, especially an ArF excimer laser featuring a shorter wavelength. There is a desire to have a microfabrication technique of finer definition by combining exposure light of shorter wavelength with a resist material having a higher resolution.
In this regard, the recently developed, acid-catalyzed, chemical amplification type resist materials are expected to comply with the deep UV lithography because of their many advantages including high sensitivity, resolution and dry etching resistance. The chemical amplification type resist materials include positive working materials that leave the unexposed areas with the exposed areas removed and negative working materials that leave the exposed areas with the unexposed areas removed.
In chemically amplified, positive working, resist compositions to be developed with alkaline developers, an alkali-soluble phenol or a resin and/or compound in which carboxylic acid is partially or entirely protected with acid-labile protective groups (acid labile groups) is catalytically decomposed by an acid which is generated upon exposure, to thereby generate the phenol or carboxylic acid in the exposed area which is removed by an alkaline developer. Also, in similar negative working resist compositions, an alkali-soluble phenol or a resin and/or compound having carboxylic acid and a compound (acid crosslinking agent) capable of bonding or crosslinking the resin or compound under the action of an acid are crosslinked with an acid which is generated upon exposure whereby the exposed area is converted to be insoluble in an alkaline developer and the unexposed area is removed by the alkaline developer.
On use of the chemically amplified, positive working, resist compositions, a resist film is formed by dissolving a resin having acid labile groups as a binder and a compound capable of generating an acid upon exposure to radiation (to be referred to as photoacid generator) in a solvent, applying the resist solution onto a substrate by a variety of methods, and evaporating off the solvent optionally by heating. The resist film is then exposed to radiation, for example, deep UV through a mask of a predetermined pattern. This is optionally followed by post-exposure baking (PEB) for promoting acid-catalyzed reaction. The exposed resist film is developed with an aqueous alkaline developer for removing the exposed area of the resist film, obtaining a positive pattern profile. The substrate is then etched by any desired technique. Finally the remaining resist film is removed by dissolution in a remover solution or ashing, leaving the substrate having the desired pattern profile.
The chemically amplified positive resist compositions adapted for KrF excimer lasers generally use phenolic resins, for example, polyhydroxystyrene in which some or all of the hydrogen atoms of phenolic hydroxyl groups are protected with acid labile protective groups. Typical photoacid generator used therein are iodonium salts, sulfonium salts, bissulfonyldiazomethane compounds, N-sulfonyloxyimide compounds, and O-arylsulfonyloxime compounds. If necessary, there are added additives, for example, a dissolution inhibiting or promoting compound in the form of a carboxylic acid and/or phenol derivative having a molecular weight of up to 3,000 in which some or all of the hydrogen atoms of carboxylic acid and/or phenolic hydroxyl groups are protected with acid labile groups, a carboxylic acid compound for improving dissolution characteristics, a basic compound for improving contrast, and a surfactant for improving coating characteristics.
A variety of phenolic resins have been developed. For improving mask linearity by restraining solubility in developer, or for imparting etch resistance following pattern formation, the recent development work deals with polymers having styrene and indene monomers copolymerized as disclosed in JP-A 8-123032, JP-A 2002-202610, JP-A 2003-84440, and JP-A 2004-115630.
A variety of photoacid generators have been developed as well. As regards the sulfonium salt, a wide variety of compounds can be developed by changing a combination of cation and anion. In particular, active research works are made on triaryl sulfonium salts because they are thermally stable and less prone to decomposition reaction by basic compounds which are added for improving the resolution of resist. As long as the present inventors have empirically found, the use of a photoacid generator which generates 2,4,6-triisopropylbenzenesulfonic acid having a bulky sulfonic acid anion is characterized by low diffusion, and the use of a photoacid generator which generates nonafluoro-1-butanesulfonic acid with strong acidity is characterized by formation of a pattern with a high resolution. As regards the photoacid generators capable of generating these sulfonic acids, the photoacid generators having 2,4,6-triisopropylbenzenesulfonate are described in JP-A 5-222257 and JP-A 10-90882, and the photoacid generators which generate nonafluoro-1-butanesulfonic acid are described in the former, JP-A 5-222257.
Studies have also been made on the sulfonium cations of the foregoing sulfonium salts. Typical attempts include the use of a photoacid generator in the form of a triarylsulfonium salt having an acid labile group substituted thereon that enables to attain a dissolution contrast in liquid developer before and after exposure, achieving a pattern profile with a higher resolution, and the introduction of a simple substituent group such as alkyl that enhances the solubility of a sulfonium salt in resist solution for providing storage stability to the resist solution comprising a specific polymer (controlling particles in liquid) as described in JP-A 9-323970, JP-A 2000-47387, and JP-A 2000-181054.
For resist materials, it is well known to combine a plurality of photoacid generators. Typical combinations include a combination of photoacid generators which generate strong and weak acids as described in JP-A 5-181279; a combination of photoacid generators which generate a fluoro-substituted alkylsulfonic acid such as trifluoromethanesulfonic acid and a non-fluoro-substituted alkylsulfonic acid as described in JP-A 8-123032; and a combination of a compound which generates a sulfonic acid having at least three fluorine atoms upon exposure to radiation with a compound which generates a fluorine-free sulfonic acid upon exposure to radiation as described in JP-A 11-72921. The inclusion of a radiation-sensitive photoacid generator based on such a combination eliminates or minimizes nano-order edge roughness or film surface roughness and improves resolution.
However, as the pattern feature size required becomes finer, even the single or combined use of these photoacid generators gives rise to several problems including low resolution, defective configuration of pattern profile due to standing waves and resist film slimming during development, instability to the ambient environment, and uneven development which causes variations in pattern feature size within the wafer plane.
The phenomenon of uneven development is discussed in detail. When a resist is coated on a wafer, exposed, baked (PEB), and developed with an alkaline developer, the time and amount of contact with the developer within the wafer plane vary with the shape of a feed nozzle and the feed method of the developer. This difference in contact with the developer can introduce a difference in pattern profile feature size although the difference introduced is of an extremely minor quantity. Specifically, where more contact with the developer is available, the resist film becomes more dissolved, resulting in a narrower pattern line width. As the pattern feature size is miniaturized, this feature size variation due to uneven development is expanded to a noticeable extent. The site subject to such a phenomenon changes with the shape of the developer feed nozzle. As the general tendency, the line width becomes narrower at the wafer center and thicker at the wafer edge.
Also, the miniaturization of the pattern feature size entails a propensity of reducing the thickness of resist film used. This gives rise to the problem that when conventional phenolic resins having acid labile groups of acetal type are used, the pattern profile becomes rounded at the top, or a satisfactory pattern profile is not obtained.